(1) Field of the Invention
The present invention relates to an alkaline battery with a nickel electrode, and particularly to the alkaline battery and the nickel electrode as well as active material for the nickel electrode.
(2) Description of the Prior Art
Alkaline batteries commonly used are called as sintered batteries, and have structures in which nickel hydroxide is loaded in a micro-porous substrate formed of a perforated steel sheet to which nickel powder is sintered. The electrode of this type requires repetition of active material loading processes for several times, resulting in complicated and thus expensive manufacturing process. Further, since the porosity of the substrate is restricted, loading density of the active material, is low, and thus an energy density of the electrode can be approximately 400 mAh/cc at most.
In attempt to improve this, electrodes other than that of the sintered type have been broadly developed. In an example of them, graphite powder of about 20-30 wt% (weight percentage) is mixed as conductive additive with nickel hydroxide powder coated with cobalt hydroxide, and then, this mixture is formed into a sheet-like shape and is fixedly pressed to a nickel plate to form the electrode. Since the above conductive additive itself does not contribute to capacity of the electrode, it reduces capacity density, and causes generation of a large amount of carbonate due to decomposition of the graphite. Therefore, this electrode can not be used in batteries having small amount of electrolyte, such as sealed nickel cadmium batteries. In order to overcome the above disadvantages, manufacturers start to provide pasted nickel electrodes for practical use, in which a metal fiber substrate having a high porosity of about 95% is used. In these electrodes, CoO powder, which forms conductive network for the active material, is added to the nickel hydroxide powder active material which is produced from nickel sulfate aqueous solution and sodium hydroxide aqueous solution, and viscous aqueous solution of carboxymethyl-cellulose is further added thereto to form paste, which is loaded in the fiber substrate. This conductive network forms current paths which are more effective than those by graphite, does not decompose, and thus does not produce carbonate. This electrode is less expensive than the sintered electrode, and has high energy density of about 500 mAh/cc.
However, as weights of portable electronics equipment have recently been reduced, the high energy density of about 600 mAh/oc is required in the market. In order to comply with this requirement, the density of the nickel hydroxide power itself must be increased, because the porosity of the substrate is limited. The nickel hydroxide powder of the high density has been used as a part of material for parkerizing steel plates. In the manufacturing thereof, nickel nitrate or nickel sulfate is dissolved in weak basic ammonia acqueous solution and is stabilized as tetra-amine nickel (II) complex ion, and the nickel hydroxide is deposited while adding sodium hydroxide aqueous solution to it. This deposition is slowly performed so as to prevent development of voids in particles.
Since this method does not perform random deposition, as is done in the conventional method it can produce the nickel hydroxide having good crystallinity with less grain boundary, i.e., less pore volume, and thus a high density.
However, due to the unique characteristics, this powder causes some problem when used as the active material for the battery as it is.
For example, the charge-discharge reaction of the nickel hydroxide electrode is performed by free movement of proton in the crystal of the nickel hydroxide. However, due to the high density of the nickel hydroxide and thus to the high compactness of the crystal, the free movement of the proton in the crystal is restricted. Further, since the current density increases in accordance with the reduction of the specific surface area, a large amount of higher oxide .gamma.-NiOOH may be produced, which may cause fatal phenomena such as stepped discharge characteristics and/or swelling. The swelling due to the production of .gamma.-NiOOH in the nickel electrode is caused by the large change of the density from high density .beta.-NiOOH to low density .gamma.-NiOOH. The inventors have already found that the production of .gamma.-NiOOH can effectively be prevented by addition of a small amount of cadmium in a solid solution into the nickel hydroxide. However, it is desired to achieve the substantially same or more excellent effect by utilizing additive other than the cadmium from the viewpoint of the environmental pollution.
Accordingly, it is an object of the invention to provide active material for nickel electrode, in which the density of the nickel hydroxide is increased, and the production of .gamma.-NiOOH, which may be caused due to the increased density, can be prevented by less poisonous additive, so that the useful life may be extended and the utilization factor of the active material may be improved. It is also an object of the invention to provide a nickel electrode utilizing said active material and an alkaline battery utilizing it.